In the NHGRI Office of the Clinical Director, Cynthia Tifft, MD, PhD, heads the Unit on Glycosphingolipid Storage Disorders, which performs translational studies on Tay-Sachs and Sandhoff diseases and GM1 gangliosidosis. Since its inception in 2009, the Unit has been actively recruiting patient with GM1 and GM2 gangliosidosis as part of a natural history study. Information gained from this study along with pre-clinical data from collaborators at the University of Massachusetts and Auburn University was used to file an IND in February, 2019 for intravenous AAV9-mediated gene therapy for patients with type II GM1 gangliosidosis. The first patient under this IND received first-in-human gene therapy in May, 2019 and an additional 5 individuals are scheduled to be enrolled by the end of the year. In basic studies, the Unit used cerebral organoids from patient-derived induced pluripotent stem cells (iPSC) to test gene therapy and small molecules to treat the fatal lysosomal storage diseases, GM1 and GM2 gangliosidoses. Specifically, the Unit is using RNA expression analysis in the cerebral organoids and brain tissue to explore the earliest perturbations in brain development resulting from mutations in HEXA and HEXB. The Unit is a recipient of two NISC pilot project grants. The first is analyzing exome sequences from a cohort of Type II GM1 gangliosidosis patients to identify variants in 1300 lysosomal-related genes as potential modifiers of disease presentation. The second project involves performing RNAseq on developing brain tissues from two patients with Tay-Sachs disease to analyze differential gene expression, and comparing it to that of normal tissues. The Unit is also phenotyping a CRISPR-generated GM1 knock out mouse that mirrors the manifestations and disease progression in patients with Type II (juvenile) GM1; this will greatly aid the lab and the field in identifying therapeutic targets for GM1 disease. Lastly, the unit is using a CRISPR-based screen for transcription factor EB (TFEB) signaling to identify genes involved in lysosome-related cell regulation. A second clinical initiative within the Office of the Clinical Director involves the NIH Undiagnosed Diseases Program (UDP), a model for Precision Medicine that provides answers to patients with mysterious conditions that have eluded diagnosis and advances medical knowledge. The UDP is part of the Undiagnosed Diseases Network (UDN), a national consortium of 12 clinical sites, a coordinating center, sequencing center, biorepository, two model organism cores, and a metabolomics core. Each year the NIH UDP reviews approximately 350 medical records and accepts 100-125 patients for thorough medical and genetic evaluations at the NIH Clinical Center. Rare disorders are diagnosed and new diseases are discovered. In 2019, UDP projects revealed the genetic bases of 7 diseases. Specifically, Saul-Wilson Syndrome was shown to be caused by a monoallelic mutation in COG4, which encodes a protein in the Conserved Oligomeric Golgi complex responsible for retrograde vesicle trafficking within the Golgi. Another study described a new disorder of developmental delay, liver abnormalities, dysmorphic features, and woolly hair associated with biallelic variants in CCDC47. A third new disease, characterized by facial dysmorphisms, congenital heart defects, and neurodevelopmental abnormalities, was associated with biallelic mutations in TMEM94. A fourth disorder described by the UDP was a combination of albinism, developmental delay, and organ storage caused by dysfunction of lysosomes and lysosome-related organelles. This occurred because of increased lysosomal acidity created by a gain-of-function mutation in CLCN7, encoding a chloride transporter that determines the influx of protons into lysosomes. Another case involved a boy with kinky hair reminiscent of Menkes disease, a copper deficiency disorder. The child had biallelic mutations in HEPHL1, whose function was determined to include oxidation of ferrous to ferric iron, allowing this metal to be incorporated into its apoenzymes. A sixth new disease involved multisystem dysfunction in a girl with a mutated SLC12A2 gene. Finally, members of the UDP described a new disorder of epilepsy, colobomas, dysmorphisms, developmental delay, and cerebellar hypoplasia due to monoallelic mutations in WDR37, which encodes a protein whose WD repeats facilitate the formation of multiprotein complexes. UDP members also assisted in the description of a new disease of developmental delay, microcephaly, and brittle nails and hair due to biallelic mutations in cysteinyl tRNA synthetase. They revealed the molecular, clinical, and biochemical bases of SLC25A2 Congenital Disorder of Glycosylation, characterizing 30 previously unreported cases of this disease, and expanded the known phenotypes of COPA syndrome, a pulmonary disorder, and late infantile ceroid lipofuscinosis, a neurodevelopmental disorder due to CLN6 mutations. UDP members reported a case of Early Infantile-onset Epileptic Encephalopathy 28 due to deletion of the WWOX gene related to uniparental disomy. One family had motor delays, coloboma and corneal defects associated with a novel TENM3 mutation, and another had dysarthria, ataxia, and sensory neuropathy related to COX20 variants. The UDP collaborated with world experts in describing atypical autosomal dominant leukodystrophy due to an upstream deletion of lamin B1; previously reported cases were due to lamin B1 duplications. These projects were pursued in collaboration with the Section on Human Biochemical Genetics in the Medical Genetics Branch of NHGRI, and are discussed in the Sections annual report as well.